1. Field of the Invention
This invention relates to ultrasound imaging and, in particular to a system including an ultrasonic probe for generating three-dimensional real images of relatively modest volumes in a body such as the eye, testes, breast, carotoid artery, tumors, kidney, liver, etc. within 5 cm of the surface.
2. Related Art
Current ultrasound systems acquire a single two-dimensional image in real-time or near-real time using an ultrasonic probe. The ultrasonic probe uses a single circularly-symmetric transducer crystal which is iterated through multiple transmit and receive cycles. Each cycle acquires a "ray" of information. When the transducer is physically moved through an arc, multiple rays are acquired at different angular positions of the transducer. The intensity of the returning echos is represented by the brightness of the corresponding pixels on a CRT screen, giving rise to the name "b-scan", or "brightness scan" for such devices. Alternatively, three-dimensional images are acquired by first taking a two-dimensional image, physically moving the entire probe, taking a second and subsequent two-dimensional image in a different plane, and merging (i.e. integrating) the multiple images together to form a three-dimensional, non-real time image. This three-dimensional image may take up to six minutes to scan and create.
Two-dimensional images are generated by a transducer, with information regarding the third dimension acquired by physical movement of the transducer. Thus, a set of static images is integrated to produce a three-dimensional image. The physical movements may take a variety of forms.
The physical movement may be linear, with a transducer mounted on a lead screw driven by a motor. Rotating the lead screw moves the transducer in a linear fashion, parallel to the surface scanned. The acquired two-dimensional images are parallel to each other, and separated by predefined spatial intervals.
The physical movement may also be a pivoting movement, also know as fan scanning. The transducer/imaging plane is rotated about an axis at the transducer face and produces an angular sweep with a fan of planes each with a predefined angular separation. The angular step between acquired planes is fixed. Accordingly, the distances between sampled regions depend on depth. The sampling distances are small near the transducer where the elevational resolution is fine. But the sampling distances are large further away from the transducer where the elevational resolution is poor.
Both the linear scanning and the fan scanning approaches are described in greater detail in the November/December 1996 issue of IEEE Engineering In Medicine and Biology Magazine, Volume 15, Number 6.
Regardless of the physical movement employed, data acquisition is limited primarily by the speed with which the transducer can be swept through its arc, and generally provides from 15 to 30 frames per second. High frame rates are desirable for visualization of dynamic processes, which includes visualization of vascular motion during tumor diagnosis, monitoring of the motion of retinal detachments and vitreous hemorrhages, and detection of foreign bodies within the vitreous. Modern B-scans provide digital storage for the display data, allowing a particular frame to be retained, or "frozen", for closer examination.
A two-dimensional image does not eliminate risk of misdiagnosis due to a lack of complete information. Similarly, a three-dimensional image which is not real time, or near real time, carries the same risk because the three-dimensional integrated image requires that the position and orientation of the two-dimensional scan plane be known for each separate image, and that the eye or other object of interest remain still for the duration of the examination. However, during ophthalmic B-scan exams, it is frequently critical that the patient move his or her eye in order for the operator to visualize the motion of the vitreous and any membranes, such as from retinal detachments, which may be present. Any such motion clearly renders static data acquisition impossible, since the position of the eye from scan to scan is then unknown.
Generally, two-dimensional B-scan data is acquired by digitizing the video output from an existing B-scan. The digitizing step necessarily requires some loss of resolution or image quality.
Alternatively, information regarding the third dimension may be acquired by replacing physical movement of the transducer by electronic scanning. Specifically, a two-dimensional array of transducers generates a pulse of ultrasound which diverges away from the array in a pyramidal shape. The echos are processed to generate three-dimensional information in real time. However, two-dimensional arrays are not yet practical because of low yields resulting from the manufacture of a large number of small elements, along with the connecting and bundling of large numbers of leads.
Accordingly, there is a need in the art to provide a near real time, or real time three-dimensional imaging system and probe capable of transmitting and receiving acoustic signals for such a system. Such a system would improve visualization of modest volumes such as posterior ocular structures and thus improve diagnosis of tumors, retinal detachments, foreign bodies, etc. In addition, since such a system would acquire volumetric images in real time or near real time, several images may be acquired in sequence to allow three-dimensional visualization of motion by stepping sequentially through the different three-dimensional images, providing even better visualization of motion of the intraocular contents.
There is a further need in the art to provide an imaging system which does not require digitizing video output. Instead, there is a need to acquired volumetric images in real-time in sequence, and store the images for processing in a three-dimensional display format which would allow three-dimensional visualization of motion by stepping sequentially through the different three-dimensional images, to provide better visualization of motion of intraocular contents.